@Dominic Mazoch posted:Another issue, maybe. Were the 2900's the heaviest Northerns built? Is that another limiting factor?
They were the heaviest 4-8-4s built - but ATSF never used the term "Northern". ATSF correspondence and blueprints refer to their 4-8-4s either by the class number (2900) or as "New Mountains", "Heavy Mountains" or "Mountains 4-Wheel Trailer". This was a consistent practice on the ATSF - note similar notations for Mikado, Pacific, and Santa Fe 4-wheel trailer. This blueprint index dated 1927 (digitized by the Kansas State Historical Society) shows 3751 class as Mountain 4 Wheel Trailer.
Here you see a blueprint of Class 3700 - Mountain Type from this blueprint book.
A copy of Blueprint for 3751 - somewhere I have another one that lists it as a "Heavy Mountain". The following are sourced from railroading online: http://www.railroadingonline.c...drawings/index.shtml